REVISION SUMMARY: Heredity (NCERT Class 10, Ch 8)
1. Chapter at a glance
- Reproductive processes produce individuals that are similar but show subtle variations; asexual reproduction yields only minor DNA-copying differences, while sexual reproduction generates greater diversity.
- Inheritance from one generation provides a common basic body design plus subtle changes; each subsequent generation inherits differences from the previous generation plus newly created variations.
- Both father and mother contribute practically equal amounts of genetic material, so each trait has two versions in the child; Mendel’s experiments with pea plants established the rules of inheritance.
- In F1 generation, only one parental trait appears (no intermediate forms); in F2 generation obtained by self-pollination, both traits reappear in a 3 : 1 phenotypic ratio.
- Two copies of the factor (gene) controlling each trait are present; the trait expressed when copies differ is called dominant, the other recessive.
- When two contrasting traits are considered together, they are inherited independently, producing new combinations in F2 (9 : 3 : 3 : 1 ratio).
- Each gene set exists as separate chromosomes; germ cells receive one chromosome from each pair, restoring the full set on fertilisation and ensuring species DNA stability.
- In humans, sex is determined genetically: females are XX, males are XY; the sex of the child depends on whether the sperm contributes X or Y.
2. Definitions and laws
- Dominant trait: a single copy is enough to express the trait (e.g., T for tallness).
- Recessive trait: both copies must be of this type for the trait to be expressed (e.g., tt for shortness).
- Gene: a section of DNA that provides information for one protein.
- Chromosome: each gene set is present as separate independent pieces called chromosomes; each cell has two copies of each chromosome (one maternal, one paternal); every germ cell takes one chromosome from each pair.
- Sex chromosomes: women have a perfect pair (XX); men have a mismatched pair (XY).
3. Important diagrams and activities
- Fig. 8.1 – Creation of diversity over succeeding generations: shows how variations accumulate and differ between asexual and sexual reproduction.
- Fig. 8.2 – Free and attached earlobes: illustrates two contrasting variants of a trait used to study inheritance patterns.
- Fig. 8.3 – Inheritance of traits over two generations: monohybrid cross showing TT, Tt (tall) and tt (short) with 3 : 1 ratio in F2.
- Fig. 8.5 – Independent inheritance of two separate traits (shape and colour of seeds): dihybrid cross demonstrating new trait combinations and 9 : 3 : 3 : 1 ratio.
- Fig. 8.6 – Sex determination in human beings: shows XX/XY inheritance pattern resulting in equal probability of boys and girls.
- Activity 8.1 – Survey of free/attached earlobes in class and correlation with parents: provides evidence for inheritance rules of a simple trait.
- Activity 8.2 – Confirmation of 1 : 2 : 1 genotypic ratio (TT : Tt : tt) in F2: verifies that both alleles are inherited even when only one is expressed.
4. Common misconceptions and exam pitfalls
- Assuming F1 plants are genetically identical to the tall parent (they are heterozygous).
- Expecting intermediate phenotypes in Mendelian crosses (NCERT states none appear).
- Thinking traits are always linked when two characters are studied together (text emphasises independent inheritance via separate chromosomes).
- Confusing phenotypic and genotypic ratios (3 : 1 vs 1 : 2 : 1).
- Believing sex is determined by the mother (text states it depends on the chromosome inherited from the father).
- Overlooking that asexually reproducing organisms also follow similar inheritance rules (text explicitly asks this question).
5. Formula sheet
No numerical formulas or equations appear in the chapter.